Some LaTeX commands

The eqn environment

Convenient maths environment

The following code defines the eqn environment, a maths environment with some convenience functions:

Code for the eqn environment

%%
%%  Math environments
%%

\ExplSyntaxOn

\cs_generate_variant:Nn \seq_set_split:Nnn { NnV }

\tl_new:N \l_content_tl
\tl_new:N \l_last_tl

\tl_new:N \l_eqn_type

\usepackage{mleftright}

\NewDocumentEnvironment{eqn}{ob}{
  \tl_set:Nn \l_tmpa_tl {#2}
  \tl_trim_spaces:N \l_tmpa_tl
  \tl_set:Nx \l_last_tl  { \tl_range:Nnn { \l_tmpa_tl } { -1 } { -1 } }
  \tl_if_in:nVTF {.,} \l_last_tl {
    \tl_set:Nx \l_content_tl  { \tl_range:Nnn { \l_tmpa_tl } { 1 }  { -2 } }
  } {
    \tl_set:NV \l_content_tl  { \l_tmpa_tl }
    \tl_set:Nn \l_last_tl {}
  }
  \tl_set:Nn \l_eqn_type {d}
  \keys_define:nn { benno/argument } {
    inline .value_forbidden:n = true,
    inline .code:n = {\tl_set:Nn \l_eqn_type {i}},
    i .value_forbidden:n = true,
    i .code:n = {\tl_set:Nn \l_eqn_type {i}},
    align .value_forbidden:n = true,
    align .code:n = {\tl_set:Nn \l_eqn_type {a}},
    a .value_forbidden:n = true,
    a .code:n = {\tl_set:Nn \l_eqn_type {a}},
    display .value_forbidden:n = true,
    display .code:n = {\tl_set:Nn \l_eqn_type {d}},
    d .value_forbidden:n = true,
    d .code:n = {\tl_set:Nn \l_eqn_type {d}},
    oneline .value_forbidden:n = true,
    oneline .code:n = {\tl_remove_all:Nn \l_content_tl {&}
                        \tl_remove_all:Nn \l_content_tl {\\}},
    autosize .value_forbidden:n = true,
    autosize .code:n = {
                        \tl_replace_all:Nnn \l_content_tl {\lvert}{\mleft\lvert}
                        \tl_replace_all:Nnn \l_content_tl {\rvert}{\mright\rvert}
                        \tl_replace_all:Nnn \l_content_tl {\lVert}{\mleft\lVert}
                        \tl_replace_all:Nnn \l_content_tl {\rVert}{\mright\rVert}
                        \tl_replace_all:Nnn \l_content_tl {\langle}{\mleft\langle}
                        \tl_replace_all:Nnn \l_content_tl {\rangle}{\mright\rangle}
                        \tl_replace_all:Nnn \l_content_tl {(|}{\mleft\lvert}
                        \tl_replace_all:Nnn \l_content_tl {|)}{\mright\rvert}
                        \tl_replace_all:Nnn \l_content_tl {(||}{\mleft\lVert}
                        \tl_replace_all:Nnn \l_content_tl {||)}{\mright\rVert}
                        \tl_replace_all:Nnn \l_content_tl {(<}{\mleft\langle}
                        \tl_replace_all:Nnn \l_content_tl {>)}{\mright\rangle}
                        \tl_replace_all:Nnn \l_content_tl {(}{\mleft(}
                        \tl_replace_all:Nnn \l_content_tl {)}{\mright)}
                        \tl_replace_all:Nnn \l_content_tl {[}{\mleft[}
                        \tl_replace_all:Nnn \l_content_tl {]}{\mright]}
                        \tl_replace_all:Nnn \l_content_tl {\{}{\mleft\{}
                        \tl_replace_all:Nnn \l_content_tl {\}}{\mright\}}
                        \tl_replace_all:Nnn \l_content_tl {\lceil}{\mleft\lceil}
                        \tl_replace_all:Nnn \l_content_tl {\rceil}{\mright\rceil}
                        \tl_replace_all:Nnn \l_content_tl {\lfloor}{\mleft\lfloor}
                        \tl_replace_all:Nnn \l_content_tl {\rfloor}{\mright\rfloor}
                        },
  }
  \IfValueT{#1}{
    \keys_set:nn { benno/argument } {#1}
  }
  \tl_log:N \l_content_tl
  \str_case:Vn \l_eqn_type {
    {i} {
      \math
      \l_content_tl
    }
    {a} {
      \equation
      \aligned
      \l_content_tl
    }
    {d} {
      \equation
      \l_content_tl
    }
  }
} {
  \str_case:Vn \l_eqn_type {
    {i} {
      \tl_if_eq:NnTF \l_last_tl {.} {
        \endmath \ifnocf{.}
      } {
        \l_last_tl\endmath
      }
    }
    {a} {
      \endaligned
      \endequation
    }
    {d} {
      \tl_if_eq:NnTF \l_last_tl {.} {
        \ifnocf{.}\endequation 
      } {
        \l_last_tl\endequation
      }
    }
  }
}

\ExplSyntaxOff

The eqn environment has the following convenience functions:

Switching between inline, displayed, and aligned maths

Using i, d, or o in the optional argument, the environment will produce inline maths (like $...$ ), a displayed equation (like \begin{equation} ... \end{equation}), or an aligned equation (like \begin{equation}\begin{aligned} ... \end{aligned}\end{equation}), respectively. No optional argument defaults to displayed maths.
Special treatment of punctuation

If the equation ends in a period or comma (. or ,) it will be treated specially. If the i is present as an optional argument, the punctuation will be moved outside of the maths environment. Furthermore, a . will only be output if there are no definitions to load (because it is assumed that every sentence will be followed by a \cfload or \cfout).
Ignoring aligment

Using oneline in the optional argument (as in \begin{eqn}[d,oneline] ... \end{eqn}) has the effect that all \\ and & in the equation will be ignored. This is useful for quickly switching an aligned equation to a displayed or inline equation. Note that this ignores all \\ and &, so it will not work well if the equation contains, e.g., a matrix.
Autosizing delimiters

Using autosize in the optional argument autosizes all paired delimiters in the equation, e.g., all (...) will be treated like \mleft(...\mright) (it works for (...), [...], \{...\}, \lvert...\rvert, \lVert...\rVert, \langle...\rangle, \lceil...\rceil, and \lfloor...\rfloor).
This can be useful if you want to quickly write down an equation without worrying about the size of delimiters and have it look decent. It's probably not a good idea to use this in a final document, though. See the tips section on delimiters on how to do this properly.

[You can actually write (|...|), (||...||), and (<...>) in place of \lvert...\rvert, \lVert...\rVert, and \langle...\rangle, respectively, but again, while this might be useful for writing something out quickly, it's not really meant for production use.]